Our invention relates to ceramic materials in general and, in particular, to those composed principally of strontium titanate (SrTiO.sub.3) and especially well suited for use as dielectrics of capacitors. Still more particularly, our invention pertains to ceramic materials of polycrystalline structure having electrically insulated boundaries between the crystal grains with a view to an increase in apparent relative dielectric constant. The invention also particularly concerns a process for the fabrication of coherent bodies of such ceramic materials.
The polycrystalline ceramics having insulated intergranular boundaries are generally referred to as the "boundary layer ceramics" in the art. An example is described and claimed in U.S. Pat. No. 3,933,668 issued to Takahashi et al. on Jan. 20, 1976. The boundary layer ceramic according to this U.S. Patent consists essentially of a major proportion of SrTiO.sub.3, a minor proportion of niobium oxide (Nb.sub.2 O.sub.5) or tantalum oxide (Ta.sub.2 O.sub.5), and a minor proportion of germanium dioxide (GeO.sub.2) or zinc oxide (ZnO). The polycrystalline ceramic of this fundamental composition has further diffused therein bismuth trioxide (Bi.sub.2 O.sub.3) or its admixture with lead monoxide (PbO) and boron oxide (B.sub.2 O.sub.3) for the insulation of the boundaries between its crystal grains.
An objection of this known boundary layer ceramic is that its crystal grains average in diameter only from about 40 to 60 micrometers. Its apparent relative dielectric constant is therefore up to 75,000 or so. Although this value is satisfactory for most intended applications of the material, there are strong demands today for ceramics with still higher relative dielectric constants.